File Transfer in the World of Slim Containers Using cURL

In the world of containers, every byte counts, the size of the image matters, and efficiency is king. But what happens when you need to transfer files out of the pods? This seemingly simple task can quickly become a headache, revealing the hidden struggles of working with slim containers.

Containers are all about efficiency—small, lightweight, and quick to start. But this focus on minimalism comes with trade-offs. One of the most significant challenges that often catches people off guard is the complexity of performing simple tasks, like transferring files between pods or external systems that are not part of the container orchestration. Examples include logs, generated data files for machine learning models or troubleshooting, or configuration files that need to be shared across multiple pods.

Lightweight Containers, Heavy Frustrations

When you're working with virtual machines (VMs), transferring files is usually a breeze. Tools like scp or ftp are readily available and make it easy to move data from one place to another. But in the world of containers, especially those built with minimalistic images, these conveniences are stripped away.

Why? Because these tools add weight to the container. And in a containerized environment, size matters—a lot. Every extra megabyte can slow down the startup time, increase the attack surface, and make scaling more cumbersome. So, in the name of efficiency, many container images leave out non-essential utilities, including those that make file transfers easy.

But what happens when you need to transfer files between pods? This is where the frustration sets in. Without scp or similar tools, you're often left scratching your head, trying to figure out how to get your files from point A to point B without compromising the lightweight nature of your containers.

The Common Workaround: A Two-Step Process

One common workaround involves copying the file from the source pod to your local machine, and then from your local machine to the destination pod. It's a clunky, time-consuming process that feels like a step backward in a world where automation and efficiency are king.

But this two-step process quickly becomes a bottleneck, adding unnecessary complexity and slowing down your pipeline.

A Solution: Streamlining Pod-to-Pod Transfers

Fortunately, there are solutions to this problem that allow you to maintain the efficiency of your containers without sacrificing functionality.

1. Using Kubernetes Volumes: One of the most straightforward solutions is to leverage Kubernetes volumes. By attaching a shared volume to multiple pods, you can enable them to read and write to the same storage, effectively bypassing the need for direct pod-to-pod transfers. While this approach works well for certain use cases, it might not be ideal for temporary or small-scale transfers.

2. Kubernetes Jobs and Init Containers: Another approach is to use Kubernetes Jobs or Init Containers. You can create a Job or Init Container with the necessary tools for the file transfer, perform the transfer, and then clean up afterward. This method ensures that your main application containers remain lean, while still allowing for more complex operations when needed.

3. Custom Scripts and Tools: If you need a more dynamic solution, consider writing custom scripts that use kubectl cp or even third-party tools designed for this purpose. These scripts can be triggered as part of your pipeline, automating the transfer process while keeping your containers streamlined.

4. Service Mesh Integration: For those looking to go a step further, integrating a service mesh like Istio can help manage and automate communication between pods, including secure file transfers. This approach requires more setup and knowledge of service mesh architectures but offers a robust solution for complex environments.

The Path Forward - A Simple Solution with curl and tar

Are these solutions still too complex for your needs? I got you covered. Here's a simple solution that can help you transfer files back and forth to external systems with SSH access without additional tools or complex configurations, by simply using curl and tar.

Step 1: Create a Tarball

First, create a tarball of the files you want to transfer. You can do this by running the following command in the source pod:

tar -czf - /path/to/files > files.tar.gz

This command creates a compressed tarball of the files you want to transfer, zipping is optional but recommended to reduce the size of the transfer.

Step 2: Transfer the Tarball - Achieving SFTP-like functionality with curl

Here's how you can upload and download a file using SFTP with curl:

Uploading a File with SFTP

curl -u <USERNAME>:<PASSWORD> -T <PATH_TO_FILE> sftp://<SFTP_SERVER>/<REMOTE_DIRECTORY>/<TARGET_FILE_NAME>

• -u <USERNAME>:<PASSWORD>: Your SFTP credentials.

• -T <PATH_TO_FILE>: The file you want to upload.

• sftp://<SFTP_SERVER>/<REMOTE_DIRECTORY>/<TARGET_FILE_NAME>: The SFTP server URL, along with the remote directory and the target file name where the file will be uploaded.

Downloading a File with SFTP

To download a file using SFTP with curl, use the -O option.

curl -u <USERNAME>:<PASSWORD> -O sftp://<SFTP_SERVER>/<REMOTE_DIRECTORY>/<FILE_NAME>

• -u <USERNAME>:<PASSWORD>: Your SFTP credentials.

• -O: Saves the file with its original name.

• sftp://<SFTP_SERVER>/<REMOTE_DIRECTORY>/<FILE_NAME>: The SFTP server URL and the path to the file you want to download.

Example Commands:

• Upload Example:

curl -u user:password -T /local/path/to/file.txt sftp://example.com/remote/path/file.txt

• Download Example:

curl -u user:password -O sftp://example.com/remote/path/file.txt

Notes:

• Make sure the curl version you are using is built with SFTP support, as not all curl versions support SFTP out of the box.

• If you need to specify a different port (other than the default port 22 for SFTP), you can do so by appending :port_number to the server address like sftp://example.com:2222/path/to/file.txt.

This approach lets you handle basic SFTP operations directly from the command line using curl. While it might not be as feature-rich as dedicated SFTP clients, it provides a simple and efficient way to transfer files without the need for additional tools or complex configurations.

Embrace the Challenge

It's easy to get frustrated when something as seemingly simple as transferring a file becomes a challenge. But it's important to remember that the limitations of containerization are a byproduct of its strengths. The very reason containers are so powerful—lightweight, efficient, scalable—is also why they sometimes feel restrictive.

In the container world, size truly does matter, but so does creativity. The limitations you face today are opportunities to innovate and streamline your processes. By understanding the pain points and exploring the solutions available, you can keep your containers lean, mean, and ready to scale—without getting bogged down by the small stuff.

So, the next time you're faced with the challenge of transferring files between pods, remember: there's always a solution that keeps your containers efficient and your workflow smooth. Embrace the challenge, and you'll find that even in the smallest containers, you can pack a powerful punch.

Happy containerizing, Go Rebels! ✊🏽